Process for preparing ceramic materials free from auto-adhesion

ABSTRACT

Process for the preparation of ceramic materials for parts having friction surfaces subject to friction and free from auto/self-adhesion under stress or during aging. A precursor of the ceramic undergoes successive operations of pressing, sintering, polishing the surface obtained, cleaning the solid gangue resulting from the polishing, roasting in the presence of oxygen, and a treatment aimed at increasing the dielectric susceptibility and homogeneity of the ceramic material adjacent the friction surfaces and to increase the mobility of charges.

The present invention relates to the field of dielectric insulatingmaterials used under friction and in particular ceramic materials.Although such materials in particular comprise organic materials such asmonolithic or composite insulants and polymers, the present inventionwill be described solely in connection with ceramics, but it must beunderstood that this only constitutes an example and must not beinterpreted as having a limitative character with respect to theapplications of the invention.

At present, there are numerous mechanical friction devices using mobileceramic elements which are in mechanical contact with one another. Thisis in particular the case with heat motors, where ceramic parts slidewith respect to one another. This is also the case with certainclutches, special bearings or running gears produced from ceramic parts.It also occurs to an ever increasing extent in the field of valves andfittings, where the ceramic material is ever more frequently used forproducing valves.

The valves of taps or faucets are formed from two generally disk-shapedparts, which have off-centered openings with respect to the tap rotationaxis and which can slide on one another. According to the relativeposition of one of the disks with respect to the other, the openingscoincide to a greater or lesser extent making it possible to control thepassage of a fluid. The surfaces of the contacting ceramic parts undergoa finishing treatment ensuring that they have a roughness and planeityappropriate for sealing the said tap. The use of ceramic parts in thisfield and in those referred to hereinbefore offers numerous advantages,including a perfect seal, an absence of wear even under a period ofprolonged operation, unalterability and very high resistance to even themost aggressive or corrosive chemical agents.

In spite of this, it is unfortunately found that the adhesion forcesbetween the ceramic parts in contact and under pressure spontaneouslyevolve as a function of numerous parameters and in particular theoperating time. In particular, a prolonged stoppage under pressure ofthe use of said parts often leads to a considerable increase in thefriction coefficient and sometimes even to seizing or total locking andthis is virtually impossible to obviate. This defect of ceramic frictionstructures, for which hitherto there has been no remedy, casts doubts onthe future of these products in such applications.

The present invention is directed at a process for the preparation ofceramic materials for friction parts free from self-adhesion phenomenaunder stress or during aging and which makes it possible to completelyand definitively obviate the disadvantages referred to hereinbefore.

This process for the preparation of ceramic materials for parts subjectto friction, in which the chemical component constituting the ceramicmaterial is subject to the successive operations of pressing, sintering,polishing the surface obtained, cleaning the solid gangue resulting fromthe polishing and roasting in the presence of oxygen is characterized inthat the preceding operations are completed by a treatment serving toincrease the dielectric susceptibility and make it homogeneous in themass in the vicinity of the friction surfaces which are to come intocontact with one another and increase the mobility of the charges.

The essential means of the present invention consisting of treating theceramics to obtain an increase in their dielectric susceptibility makingit homogeneous in the mass has resulted from recent theoretical researchwhich has revealed that the auto/self-adhesion, seizing and lockingphenomena previously observed had an electrostatic origin.

The electrostatic theory of adhesion is based on the calculation of thepressure due to the presence of electrical charges localized at theinterface of the insulating materials having different dielectricsusceptibilities. It has experimentally been shown that a polarizationenergy accumulates in any insulating material around faults or defects(in the sense of the physics of solids) under the effect either of anelectrical stress, or a mechanical stress. The higher the polarizationenergy, the greater the adhesion force. The said energy can bedissipated by relaxation with or without modification of the dielectricand this modification can extend to the destruction of the material.

Thus, the dielectric and mechanical properties of insulating materialsare naturally correlated, because in two different forms they representthe properties of the material. The microscopic parameters leading tothese properties are dependent on the dielectric susceptibility of thecontacting materials, as well as the nature and density of the defects.

The solution for improving the non-adhesion properties consequentlyconsists of a better distribution of the defects within the material, sothat their density around the contact zones is as low as possible andconsequently the charges trapped on these defects are distributed withinthe mass. This is the aim of increasing the susceptibility recommendedin the process according to the invention.

According to a first feature of the ceramic material preparation processaccording to the invention, the dielectric susceptibility increase isobtained by doping the insulant with the aid of at least one metaloxide, which is preferably chosen from among manganese and titaniumoxides. These two oxides have electrochemical properties making itpossible for them, as soon as they are inserted by doping in the ceramicmass, to increase the mobility of the charges trapped in the potentialwells of the material.

According to another feature of the process according to the invention,said same ceramic susceptibility increase is obtained by irradiationwith the aid of ionizing rays. The ionizing rays, which are e.g. usuallygamma or X-rays, increase the number of defects present in the vicinityof the surfaces in question and increase the mobility of the electriccharges trapped in the potential wells surrounding the defects.

The intensity of the treatments performed for curing the surfaces oftheir natural tendency to increase the friction coefficient is left tothe evaluation of the expert, bearing in mind that the present inventionalso proposes a process for checking the production of the thus obtainedceramics. This process is characterized in that the thus treatedmaterials are examined with an electron microscope used as theelectrostatic potential measuring probe and in that in this way thepermittivity of the ceramic is determined, as well as the distributionof the defects and the electric power dissipatable in the dielectricmedium constituting the ceramic without causing any deterioration of thelatter.

In practice, the preparation of a ceramic free from an auto-adhesionphenomenon requires a first treatment phase, followed by a process ofestimating the result obtained and then a complementary treatment and afurther check may then be necessary. Thus, case by case, the expert willbe able to monitor production in order to bring it to the point wherethe ceramic definitively has the desired properties.

An embodiment of the process according to the invention will now bedescribed in an illustrative and non-limitative manner with respect toits performance conditions.

Consideration will be given to the case of industrial aluminas used fortap or faucet seals and shaped by conventional pressing, moulding andsintering processes. A polishing operation removes a material thicknessof approximately 10 microns. Following the polishing operation theceramic disks must have the requisite roughness and planeity in order toensure the necessary seal. However, the surface is covered by a solid,thick and adhesive gangue formed from all the polishing products andwaste materials. The gangue is cleaned in successive baths (differentpH-values, temperature 200° C., ultrasonics). A final treatment,generally roasting in air or under oxygen at about 1500° C., makes itpossible to saturate the bonds which might be available and relax theresidual stresses. All these operations are known to the ceramic expert.

According to the invention, there is a modification of the distributionof the surface and volume defects remaining after machining, by dopingand/or irradiation.

Doping increases the dielectric susceptibility of the surface layers.Thus, susceptibility is a macroscopic view of the defects. In order tomodify it, diffusion takes place of one or more metal oxides. Examplesare manganese or titanium oxide. The temperature and duration of thediffusion are determined with the aid of the electrostatic methoddescribed hereinafter. For example, a dielectric susceptibility gradientoptimum was obtained by the diffusion of titanium oxide at 1300° C. forthree minutes. With this doping, it is found that the friction noise ismuch lower, that there are no shaking effects and that friction isnormal on starting. On reaching this production stage, it is necessaryto check the production process.

On the basis of previously established correlations between the adhesionproperties and the electrical properties, the treatment is optimized byelectrostatic checks. This is an extremely sensitive, rapid and highperformance process. It avoids the conventional mechanical operationsnormally consisting of measuring the friction coefficient after a largenumber of passages plane on plane.

Electrostatic production checks or inspections take place with ascanning electron microscope (SEM). The process consists of electricallycharging an area of the insulating sample with the SEM beam at thenominal energy of the apparatus (e.g. 50 keV), followed by theobservation of the irradiated area with the low energy SEM beam, e.g.0.3 keV. The charged area reflects the beam and there is a "mirroreffect". The measurement of the dimensions of the mirror as a functionof the observation voltage makes it possible to plot a curve, whosegradient is correlated with the distribution of the defects andtherefore the adhesion properties of the material. In this way thepermittivity of the ceramic is determined, as well as the distributionof the defects and the electrical power which can be dissipated in theceramic without leading to its deterioration.

The optimum of a treatment (doping or irradiation) of the ceramic isobtained when the gradient of the curve reaches a maximum. Any standardcommercial apparatus can be used, provided that it is equipped with ameasuring system for the sample current and that it can operate at verylow voltages.

We claim:
 1. Process for the preparation of ceramic parts includingfriction surfaces having reduced auto-adhesion under stress and duringaging, said process including the steps of:(a) pressing ceramic materialto form parts having said friction surfaces; (b) sintering the ceramicmaterial from step (a); (c) polishing said friction surfaces of thesintered material from step (b) with the formation of solid gangue; (d)cleaning said friction surfaces to remove said solid gangue resultingfrom the polishing in step (c); (e) roasting the cleaned frictionsurfaces from step (d) in the presence of oxygen; and (f) irradiatingthe roasted surfaces of from step (e) with x-rays at a dose of about1000 Rad to provide said ceramic materials with increased dielectricsusceptibility and homogeneity adjacent said friction surfaces. 2.Process for checking the manufacture of ceramic materials according toclaim 1 wherein said ceramic material from step (f) has a permittivity εdefect distribution and dissipatable level of electrostatic powerwithout deterioration thereof, and including the steps of determiningthe permittivity, defect distribution and dissipatable electrostaticpower by measuring the electrostatic potential of the ceramic materialusing a scanning electron microscope.
 3. Process for checking themanufacture of ceramic materials according to claim 2, including thesteps of charging an area of the ceramic material from step (f) with arelatively high energy beam from said scanning electron microscope toform local electrical fields, applying a relatively low energy beam fromsaid scanning electron microscope onto said charged area, and sensingthe deflection of said low energy beam with said scanning electronmicroscope.